In the process of color lithographic printing, as methods for obtaining a color proof from plural sheets of black-and-white halftone dot images obtained by color separation and halftone dot image conversion, the overlay method and the surprint method are conventionally known, which are methods of forming color images by the use of a photopolymer or a diazo compound.
The overlay method is advantageous in that it is very simple, can enjoy a low production cost and can be used to carry out proofing only by overlaying four-color (subtractive primaries and black) film sheets. It, however, is disadvantageous in that the overlaying of the film sheets causes a gloss, which gives a texture different from that of a print.
The surprint method is a method in which colored images are superposed on the same support. This method is known to include methods in which the colored images are obtained by toner development that utilizes adhesive properties of a photopolymerizable material, as disclosed in U.S. Pat. Nos. 3,582,357 3,607,264 and 3,620,726.
Another method is known in which transfer is made on a support by the use of a light-sensitive coloring sheet followed by exposure and development to form an image, and thereafter another coloring sheet is superposed thereon, followed by repetition of the like process to prepare a color proof, as disclosed in Japanese Patent Examined Publication No. 27441/1972 and Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 501217/1981.
A still another method is known in which, using light-sensitive coloring sheets, corresponding color separation films are exposed and developed and the resulting respective colored images are transferred and formed on the same support, as disclosed in Japanese Patent O.P.I. Publication No. 97140/1984. As toners and as colorants for the coloring sheets, used to form these images, the same coloring materials as used in printing inks can be used. Because of this advantage, the resulting color proof can have a color tone resembling that of a print.
In these methods, however, images must be superposed and transferred in the process of preparing a color proof, and there are disadvantages that operation therefor takes a long time and they require a high production cost.
As methods that have eliminated such disadvantages, a method of preparing a color proof by the use of a silver salt color photographic material having a white support is disclosed in Japanese Patent O.P.I. Publication Nos. 113139/1981, No. 104335/1981, No. 280746/1987, No. 280747/1987, No. 280748/1987, No. 280749/1987, No. 280750/1987 and No. 280849/1987.
In this method, plural sheets of color-separated black-and-white halftone dot images, converted into halftone dot images color-separated from a color original, are successively photographically printed on a sheet of color photographic paper by contact printing or the like, followed by color development, and a color image formed of color dyes imagewise produced from couplers as a result of the color development is used as a proof image.
This technique, however, has a disadvantage that an attempt to make a color image resemble a print results in an insufficiency in the density of black images such as characters compared with that of the print and on the other hand any means taken to increase this density so as to make black image such as characters have a density resembling that of a print brings about a low resemblance of the print to the color image, making it difficult to satisfy the both at the same time.
As a technique to overcome this disadvantage, it is conventionally known to provide, in addition to a yellow color forming layer, a magenta color forming layer and a cyan color layer, a fourth black layer having a spectral sensitivity different form that of any other layer. Japanese Patent O.P.I. Publications No. 289846/1990 and No. 183251/1990 also disclose techniques that can solve this problem. All of them, however, still have the problems such that color tones may vary when photographic processing is ordinarily carried out and color tones of the resulting images may have an insufficient stability, and a further improvement is sought in order for the techniques to be put into practical use.
With regard to a reflective support comprising a base paper provided on each side thereof with a polyolefin resin coat layer, it is disclosed, for example, in Japanese Patent O.P.I. Publications No. 46035/1979 and No. 28640/1990 to increase the amount of titanium dioxide incorporated in the polyolefin resin coat layer on the side on which photographic emulsions are coated. There, however, is provided no black image forming layer, and nothing is mentioned about the problem of the variations of color tones that may occur when photographic processing is ordinarily carried out.